On the literal side, John calls the boxes "caskets" because that is a word that he knows that seems to fit them. A casket (in Shakespeare) is a small box, not a coffin like it is in our modern language.

Figuratively, though, I think that Huxley uses this term because of the meaning that we as modern readers give it. When we hear the word "casket" we think of death. By using the word, Huxley implies that there is death in those boxes. Soma is, in a way, death. It helps to kill the humanity of the residents of the brave new world. It dulls their minds and removes their emotions.

This is, I believe, why Huxley has John use that word for the boxes in Chapter 11.